1. Field of the Invention
The present invention relates to indicators for denoting the condition of an electric fuse. More specifically it relates to an indicator which is mounted on the outside of the fuse in parallel with the current path of the fuse.
2. Description of the Prior Art
A well-known type of current limiting fuse comprises a housing of insulating material having opposite ends closed by a metal cap or ferrule, commonly referred to as an end terminal. At least one fusible element extends through the housing and is electrically conductively connected to the end terminals. During normal current conditions, the fusible elements remain intact. When certain abnormal current conditions occur, the fusible element melts and interrupts the flow of current through the circuit the fuse is protecting. Arcing normally occurs during the normal interruption operation of a fuse and for this reason the housing is usually filled with a granular arc-quenching filler such as sand.
When a fuse has operated in response to abnormal circuit conditions, to successfully interrupt a circuit, there is normally no obvious change in the outward appearance of the fuse. For that reason, many fuses currently in use are provided with an indicator which is designed to operate when the fuse operates to provide a visual or otherwise a ascertainable indication that the fuse has opened.
Some examples of fuses having indicators are shown in U.S. Pat. Nos. 3,535,668, 3,593,246, 3,601,739 and 4,023,133. Such indicators are normally retained in a retracted position, against the force of a spring, by a restraining wire. The restraining wire usually extends throughout the length of the fuse casing and is in electrical contact with the terminals at the opposite ends thereof.
When a fuse is operating under normal conditions, the indicator wire does not conduct a significant current flow because it has a higher ohmic resistance than the fusible elements. When the fusible elements have blown, the restraining wire conducts current momentarily until it likewise melts or fuses thereby releasing the indicator and allowing the spring to move it into a position signifying that the fuse has blown.
Another type of blown-fuse indicator is shown in U.S. Pat. Nos. 3,179,774 and 3,621,431. These indicators have the appearance of a fuse but actually contain a spring loaded indicator pin normally restrained by a restraining wire. They are meant to be wired in parallel with the fuse being monitored. Often this type of indicator is mounted remote from the fuse.
It is an object of the present invention to provide a blown fuse indicator which may be optionally attached to appropriate mounting tabs on the side of an electric fuse.
It is another object of this invention to provide an externally mounted blown fuse indicator which does not require compromising the structural integrity of the fuse casing.
A related object is to provide an externally mounted blown fuse indicator which has an indicating pin that extends perpendicular to the longitudinal axis of the fuse.
It is a further object of the resent invention is to provide a simple, low cost, reliable blown fuse indicator.